The present invention relates to a fiber finish composition, a process which includes treating polyester yarn therewith and polyester yarn so treated. More particularly, the present invention relates to a process for producing adhesive active polyester, preferably polyethylene terephthalate, yarn wherein the yarn is heated to a temperature of about 150.degree. C. to 230.degree. C. and treated with a fiber finish composition which comprises: a triglycidyl ether of glycerol; a low viscosity diglycidyl ether having the structural formula ##STR3## wherein R is alkylene or arylene; ethoxylated castor oil; an epoxy silane having the structural formula ##STR4## wherein n=2 to 5; and a solvent.
The yarn is subsequently twisted into tire cords for the construction of pneumatic passenger tires and results in excellent adhesion of tire cord to rubber.